1. Field of the Invention
This invention describes a manufacturing process for producing large physical area arrays of discrete components, more particularly to electro-photographic methods which can produce arrays of small components such as transistor circuits useful, for example, for flat panel displays.
2. Description of the Related Art
The most common flat panel display is the Active Matrix Liquid Crystal Display, typically used in lap top computers. In an active matrix each “pixel” location has a functioning transistor that switches “On” and “Off” the voltage to that pixel. The transistor modulates the voltage to achieve levels of brightness for continuous “tones” in the picture. The manufacture of the active array of transistors is a time-consuming, expensive task. Beginning with an X-Y grid of metallized conductors and transparent electrodes, the transistors are formed in a layered pattern using sputtering and photolithography. The lithographic steps consist of:                A). spin coating an etch resist        B). soft bake to harden resist        C). photo expose        D). develop: i.e., wash away unexposed resist        E). cure remaining resist by baking        F). etch material underneath        G). strip away the resist.Between steps B and C the substrate must be cooled back to room temperature otherwise thermal expansion will result in mechanical alignment errors.        
The formation of the simple transistor, usually a Field Effect Transistor (FET) requires the following layers, each of which is photolithographically patterned:                1. Amorphous silicon layer        2. Silicon nitride dielectric layer        3. Adhesion layer        4. Metallization layer of Aluminum,Two of the layers (1 and 3) are formed in an expensive vacuum sputtering chamber, while layer 4 is formed in a somewhat less expensive vacuum thermal evaporation process. Each of the layers is patterned by the 7 step photolithographic process previously described for a total of 35 steps.        
There is a desire for flat panel displays to be very large, perhaps one meter in diagonal. The above process is costly for very small panels, to scale it to “meter” dimensions has proved difficult despite $10's of billions and two decades of research worldwide.
Part of the difficulty of the above process is that the quality of the amorphous silicon is marginal, at best. It has very low “minority carrier mobility” (carrier velocity per unit of electric field) compared to the single crystal silicon (from wafers) that is used to make all other integrated circuits. To this end, an improvement could be effected if the amorphous silicon could be crystallized resulting in substantial improvements in performance. If the amorphous silicon can be crystallized and transformed to “Poly Si”, then other circuits can be incorporated into the display beyond the transistor at each “pixel” element (e.g., the “Housekeeping” circuits like row and column drivers, data shift registers, etc.). This would also substantially reduce the interconnects required and lower the cost.
Substantial research has been executed attempting to produce such “poly” silicon. High powered lasers “Zap” the amorphous silicon, raising its temperature thereby crystallizing it. Solid phase crystallization through the agent of a catalyst like palladium or other metal with heating to 600° C. for 10 minutes has been disclosed (see Fonash, U.S. Pat. No. 5,275,851). But none of the “poly Si” processed have been successfully commercialized.
Many people have proposed mounting “real silicon” die, cut from a silicon wafer. These parts may be as small as 100×100 microns in dimension. They would be very inexpensive themselves; however, the problem would be in applying them at each fixed location. In a very high resolution display there may be as many as 3 million pixels, each of 3 colors, for a total of 9 million “pixels” meaning 9 million transistors that need to be properly and precisely located.
One approach to this problem is that of Alien Technology of Hayward, Calif. Alien describes die that are fabricated with beveled rear faces and are “fluidically” located in a preformed back plate, to “micron tolerances”. This back plate is part of the final product and is shipped “out the door” to the customer. Furthermore there are no “imaging forces” that move the “die” into the preformed cavities. The only force available is that of gravity.
There remains the need to assemble discrete active silicon parts; cheaply and massively that this invention is presented.
The basic technology of “electrophotography” or images made by electrical forces is approximately 50 years old and generates approximately $100 billion in revenue, worldwide. Its legacy includes the copy machine and the laser printer. In virtually all manifestations the machines put a colored resin toner on a sheet of paper for a human being to read. In recent years the Electrox Corp. of Denville, N.J. has pioneered the use of the technology as a manufacturing tool; printing functional materials configured as liquid toners, on metal, glass and even paper surfaces. Toners include Metals(Ag, Al); Catalysts (Pb, Sn); Dielectrics (Glass, Phosphors); and Resins (Etch resists, Color Filter Materials, Organic LED Materials).